1. Field of the Invention
The present invention pertains generally to extreme ultraviolet lithography (EUVL), and more particularly to at-wavelength inspection of reflective multilayer-coated mask blanks.
2. Description of the Background Art
Reflective extreme ultraviolet light (EUV) mask blanks, which are fabricated and subsequently patterned, are becoming critical elements in a relatively new photolithographic technology that incorporates EUV light. Mask blanks are formed from large (e.g., typically greater than four-inch square or diameter) substrates coated with a spatially uniform, reflective multilayer structure. The fabrication tolerances for lithographic quality mask blanks require reflectivity uniformity in the range of ±0.1%, and centroid wavelength variation of a few tenths of an angstrom. The success of this technology therefore relies on the development of metrology tools capable of accurately and efficiently measuring large numbers of mask blanks, both for the qualification of the mask blanks and the tools that produce them.
The characterization of a large number of EUV mask blanks has been an essential aspect of the calibration of various multilayer deposition tools, and in the cross-correlation of different multilayer characterization systems. In the current state of the art, when the coating uniformity is to be assessed, a narrowed EUV beam is focused to a relatively small spot on the mask. A suitable detector or detectors measures the power of the reflected beam, from which the reflectivity is calculated. During the measurement, a number of parameters may be varied, including the position of the measurement, the angle of incidence and the wavelength of the illumination. From these measurements, the multilayer d-spacing, Γ parameter, and centroid- or peak-of-reflectivity wavelength may be determined.
The spatial dependence of the multilayer parameters is measured one spot at a time, in series. The size of the radiation spot on the mask blank controls, to some extent, the spatial resolution of the measurement. Where the spot is small, concern may arise that a sparse sampling of measurement positions on the mask may miss significant features. On the other hand, where the spot is relatively large, it may average together high- and low-quality regions producing results that may not characterize the sample well.
Therefore, there is a need for a method and apparatus for characterizing a large number of EUV blanks that is well suited for inspection of moderate and low spatial frequency coating uniformity. The present invention satisfies those needs, as well as others, and overcomes deficiencies in conventional approaches.